Field of the Invention
The present invention relates to an insulated synchronous rectification DC/DC converter.
Description of the Related Art
Various kinds of consumer electronics devices such as TVs, refrigerators, etc., each operate receiving commercial AC electric power from an external circuit. Also, electronic devices such as laptop computers, cellular phone terminals, and tablet PCs are each configured to operate using commercial AC electric power, and/or to be capable of charging a built-in battery using such commercial AC electric power. Such consumer electronics devices and electronic devices (which will collectively be referred to as “electronic devices” hereafter) each include a built-in power supply apparatus (AC/DC converter) that performs AC/DC conversion of commercial AC voltage. Alternatively, in some cases, such an AC/DC converter is built into an external power supply adapter (AC adapter) for such an electronic device.
FIG. 1 is a block diagram showing an AC/DC converter 100r investigated by the present inventor. The AC/DC converter 100r mainly includes a filter 102, a rectifier circuit 104, a smoothing capacitor 106, and a DC/DC converter 200r. 
The commercial AC voltage VAC is input to the filter 102 via a fuse and an input capacitor (not shown). The filter 102 removes noise included in the commercial AC voltage VAC. The rectifier circuit 104 is configured as a diode bridge circuit which performs full-wave rectification of the commercial AC voltage VAC. The output voltage of the rectifier circuit 104 is smoothed by the smoothing capacitor 106, thereby generating a converted DC voltage VIN.
An insulated DC/DC converter 200r receives the DC voltage VIN via an input terminal P1, steps down the DC voltage VIN thus received, and supplies an output voltage VOUT stabilized to the target value to a load (not shown) connected to an output terminal P2.
The DC/DC converter 200r includes a primary-side controller 202, a photocoupler 204, a feedback circuit 206, an output circuit 210, a synchronous rectification controller 300r, and other circuit components. The output circuit 210 includes a transformer T1, a diode D1, an output capacitor C1, a switching transistor M1, and a synchronous rectification transistor M2. The output circuit 210 has the same topology as those of typical synchronous rectification flyback converters, and accordingly description thereof will be omitted.
The switching transistor M1 connected to the primary winding W1 of the transformer T1 performs switching so as to step down the input voltage VIN, thereby generating the output voltage VOUT. With such an arrangement, the primary-side controller 202 adjusts the duty ratio of the switching of the switching transistor M1.
The output voltage VOUT of the DC/DC converter 200r is divided by means of resistors R1 and R2. The feedback circuit 206 includes a shunt regulator or an error amplifier that amplifies the difference between the divided voltage (voltage detection signal) VS and a predetermined reference voltage (VREF) (not shown), and generates an error current IERR that corresponds to the difference, which is drawn (as a sink current) via a light-emitting element (light-emitting diode) arranged on the input side of the photocoupler 204.
A feedback current IFB flows through a light-receiving element (phototransistor) on the output side of the photocoupler 204 according to the error current IERR that flows on the secondary side. The feedback current IFB is smoothed by means of a resistor and a capacitor, and is input to a feedback (FB) terminal of the primary-side controller 202. The primary-side controller 202 adjusts the duty ratio of the switching transistor M1 based on the voltage (feedback voltage) VFB at the FB terminal.
The synchronous rectification controller 300r switches on and off the synchronous rectification transistor M2 in synchronization with the switching of the switching transistor M1. The synchronous rectification controller 300r includes a pulse generator 304 and a driver 306. The pulse generator 304 generates a pulse signal S1 in synchronization with the switching of the switching transistor M1. For example, when the switching transistor M1 turns off, the pulse generator 304 sets the pulse signal S1 to a first state (e.g., high level) configured as an instruction to turn on the synchronous rectification transistor M2. When the secondary current IS that flows through the secondary winding W2 becomes substantially zero in an on period of the synchronous rectification transistor M2, the pulse generator 304 sets the pulse signal S1 to a second state (low level) configured as an instruction to turn off the synchronous rectification transistor M2.
During the on period of the switching transistor M1, the voltage across both ends of the secondary winding W2 is represented by (−VIN×NS/NP). Accordingly, the drain voltage VD (i.e., drain-source voltage VDS) of the synchronous rectification transistor M2 is represented by (VOUT+VIN×NS/NP). Here, NP and NS represent the number of turns of the primary winding W1 and the number of turns of the secondary winding W2, respectively.
When the switching transistor M1 turns off, the secondary current IS flows from the source to the drain of the synchronous rectification transistor M2. In this state, the drain-source voltage becomes a negative voltage. In the continuous mode, when the switching transistor M1 turns on, the secondary current IS becomes zero. In this stage, the drain voltage VD jumps up again to a value represented by (VOUT+VIN×NS/NP). In the discontinuous mode, when the synchronous rectification transistor M2 turns on, the energy stored in the transformer T1 decreases. In this state, the secondary current IS also decreases, which reduces the absolute value of the drain-source voltage VDS. Eventually, the secondary current IS becomes substantially zero. In this stage, the drain-source voltage VDS also becomes substantially zero. In this state, ringing occurs in the drain voltage VD.
Using this mechanism, the pulse generator 304 generates the pulse signal S1 based on the drain voltage (drain-source voltage) of the synchronous rectification transistor M2.
The driver 306 switches on and off the synchronous rectification transistor M2 according to the pulse signal S1. The above is the overall configuration of the AC/DC converter 100r. 
The present inventors have come to recognize the following problems that occur in the DC/DC converter 200r shown in FIG. 1 when it operates in the continuous mode.
FIG. 2 is an operation waveform diagram showing the operation of the DC/DC converter 200r shown in FIG. 1 when it operates in the continuous mode. Before the time point t1, the switching transistor M1 turns on. In this state, the drain voltage VD of the synchronous rectification transistor M2 is represented by (VOUT+VIN×NS/NP). When the transistor M1 is turned off at the time point t1, the secondary current IS starts to flow through the secondary winding W2. In this state, the drain voltage VD becomes a negative voltage. When the pulse generator 304 detects that the drain voltage VD crosses a first threshold voltage VTH1 when it drops from the upper side to the lower side, the pulse generator 304 sets the pulse signal S1 to a first state. As a result, the synchronous rectification transistor M2 is turned on.
In the on period of the synchronous rectification transistor M2, the absolute value of the drain voltage VD decreases according to a reduction in the secondary current IS. When the switching transistor M1 turns on at a time point t2, the secondary current IS becomes zero. In this state, the drain voltage VD jumps up again to a value represented by (VOUT+VIN×NS/NP). When the drain voltage VD crosses a second threshold voltage VTH2 when it rises from the lower side to the upper side, the pulse generator 304 sets the pulse signal S1 to a second state. As a result, the synchronous rectification transistor M2 is turned off
With such an arrangement, there is a delay time period τD from the time point t2 at which the drain voltage VD crosses the threshold voltage VTH2 up to a time point t3 at which the synchronous rectification transistor M2 turns off according to a transition of the pulse signal S1 to the second state. During the delay time τD, the synchronous rectification transistor M2 turns on. In this state, a large voltage VD occurs across the synchronous rectification transistor M2 in a state in which it has an extremely low impedance. Accordingly, in some cases, this leads to a problem of a large amount of current flowing through the synchronous rectification transistor M2 (as indicated by the broken line IS′).
During the delay time τD, the large current IS′ flows through the synchronous rectification transistor M2 via the secondary winding W2. When the synchronous rectification transistor M2 turns off at the time point t3, the current IS′ that flows through the secondary winding W2 is cut off. This generates a high voltage across both ends of the secondary winding W2 as represented by Vx=dIS′/dt. The high voltage Vx induces the voltage Vy across both ends of the primary winding W1 as represented by Vy=−Vx×NP/NS. In a case in which the voltage Vy thus induced is applied to the switching transistor M1, in some cases, this leads to degradation of the reliability of the switching transistor M1.
In order to solve such problems, an approach is conceivable in which the primary-side controller 202 supplies, to the synchronous rectification controller 300, a timing signal which indicates the turn-on of the switching transistor M1. With such an arrangement, the synchronous rectification controller 300 turns off the synchronous rectification transistor M2 before the turn-on of the switching transistor M1.
However, with such an insulated converter, there is a need to provide electrical insulation between the primary side and the secondary side. Such an arrangement requires an additional photocoupler or a capacitor, leading to a problem of an increased circuit cost.